This invention relates generally to magnet systems for generating high-frequency fields, and more particularly, to a high-frequency magnet system for use in an image-producing apparatus which operates under the principles of nuclear magnetic resonance technology, such as zeugmatography, and which uses a pair of magnet coils which are arranged on an imaginary cylindrical surface for generating an essentially homogeneous high-frequency magnet field in a direction which is radial with respect to the axis of the imaginary cylindrical surface.
In the field of medical diagnostics, imaging methods have been proposed wherein an image which is similar to an X-ray tomogram is constructed from the spatial spin density and/or relaxation time distribution by using either numerical or measurement analysis of integrated proton resonance signals. A system for practicing such zeugmatography, or nuclear spin tomography, is described in the publication "Nature," volume 242, 1973, pages 190-191.
Nuclear magnetic resonance is applied by placing the body to be examined in a strong, homogeneous magnetic field which, as noted above, extends in a direction which is radial with respect to the axis of the imaginary cylindrical surface on which the magnet coils are arranged. One such magnet system is described in European patent application EP 21 535 A1. The magnetic field which is produced by such a magnet system serves as a base field, which, with respect to the body to be examined, will be assumed to be oriented in the z-direction in an x, y, and z coordinate system. Stationary and/or pulsed gradient fields are superimposed on the base field, which shall be designated B.sub.OZ. In addition, an alternating high-frequency field must be provided and arranged perpendicular to the base field. This alternating high-frequency field should be as homogeneous as possible. The alternating time-variable high-frequency field is represented by B.sub.1 =B.sub.X e.sub.X cos .gtorsim.t, and is oriented, for example, in the x-direction so as to have an amplitude B.sub.X in the direction of the unit vector e.sub.X. This alternating magnet field serves to tilt the nuclear spin magnetization of the body to be examined, which may be human, from the z-direction to the x-y plane. The x, y, component of the magnetization so produced precesses about the z-axis and induces in a pickup coil which is aligned in the x or y direction an electric signal which is amplified and evaluated. Instead of a separate pickup coil, the high-frequency exciter coil can also be provided for detection. Only the component which rotates in the same sense as the spin precession is physically effective for exciting the nuclear spin magnetization.
In zeugmatography, the signal-to-noise ratio of the signal induced in the pickup coil increases with the base field B.sub.O =2.pi.f/.gamma., where f is the corresponding precession frequency of the nuclear spin, and .gamma. is the gyro-magnetic factor of the protons. For protons, for example, this factor is .gamma.=2.pi..multidot.42.57 MHz/Tesla. Since the quality of the pictures produced depends on the signal-to-noise ratio, it is desirable to obtain base fields B.sub.OZ as high as possible and thereby high-frequencies F=.gamma.B.sub.OZ /2.pi., as described in J. Phys. E.:Sci. Instrum., vol. 13, 1980, pages 38-44. In known zeugmatographic equipment, such as that described hereinabove in EP 21 535 A1, only relatively low frequencies of about 5 MHz can be provided for physical reasons, because at higher frequencies, eddy currents are observed which are induced in the electrically conductive material of the body to be examined. In this respect, tissues of live biological bodies having a conductivity EQU .delta.=0.5[ohm.multidot.m].sup.-1
of can be considered as electrically conductive. The eddy currents weaken the high-frequency field in the interior of the body because of the skin effect, and render the field nonhomogeneous. Thus, it is a problem with the known systems that the signal-to-noise ratio cannot be increased by raising the frequency of operation of the known systems for determining nuclear spin resonance.
It is, therefore, an object of this invention to provide an image-generating system which utilizes nuclear magnetic resonance technology of the type mentioned herein in such a manner that a relatively large signal-to-noise ratio is achieved, thereby enabling the use of high frequencies, illustratively 20 MHz.